Pure speculation on my part......people make decisions everyday based on what they see in Microsoft products. Now, in an ever increasing manner people are making decisions on what they see in future Microsoft products (Whidbey, etc.). It would be a bad thing for Microsoft if someone were to see X# and make decisions based on its current state as well as the idea that it may be a product someday when in fact it may not. This scenario is much like Microsoft releasing the product specification for something they've just begun to build. There are lots of things that get cut from products throughout the life cycle for various reasons and it is usually better that folks find out the features that will ship with the product rather than the sky high expectations of the team (or in this case a research group).
I'm sure competition is something that would be a factor as well.